1. Field of the Invention
The present invention relates to an SDH multiplex transmission system and apparatuses for the same. In particular, the present invention relates to an SDH multiplex transmission system having a transmission terminal apparatus and a reception terminal apparatus that are connected to each other through channels of an N+1 redundant structure.
Recently, many communication networks have begun to employ an SDH (Synchronous Digital Hierarchy), and not only optical transmission apparatuses but also radio transmission apparatuses are shifting from PDH (Plesicohronous Digital Hierarchy) to SDH.
2. Description of the Related Art
FIG. 14 shows the structure of a PDH digital multiplex radio transmission system according to a prior art.
In the figure, terminal apparatuses 300 and 400 are connected to each other through radio channels with or without a relay (not shown) between them. The terminal apparatuses 300 and 400 have each a pair of transmission and reception terminal apparatuses 300T (400T) and 300R (400R) for each upward and downward channel, to realize two-way communication.
In the transmission terminal apparatus 300T, TBBIF is a transmitting terminal baseband interface, TSW is a transmit switch for radio protection, MOD is a modulator, TX is a transmitter, TBSC is a transmitting terminal baseband switch control, TTCU is a transmitting terminal timing control unit, and PG is a pattern generator. In the reception terminal apparatus 400R, RX is a receiver, DEM is a demodulator, RSW is a receive switch for radio protection, RBBIF is a receiving terminal baseband switch control, RBSC is a receiving terminal baseband switch control, RTCU is a receiving terminal timing control unit, and PD is a pattern detector.
The system of this type generally employs an N+1 (2+1 in the figure) redundant structure. If a current channel MCH1 or MCH2 causes a communication fault due to, for example, fading, the current channel is switched to a spare channel PCH to continue communication. The switching time must keep a level of 10 msec to follow a fading speed. Even if a channel is switched to another, frame synchronization between the transmission and reception terminal apparatuses must be maintained. To achieve this, the data transmission phase of each channel is adjusted according to respective frame synchronization signals FP.
More precisely, a main signal MCH1 (represented with the same mark as the current channel MCH1) in the upward channel is stored in an elastic buffer (not shown) in the interface TBBIF1 according to a clock signal CLK' that has been received and regenerated. Thereafter, the stored signal is sequentially read in synchronization with the frame synchronization signal FP of the terminal timing control unit TTCU according to a clock signal CLK. The signal MCH1 is passed through the switch TSW1 to the modulator MOD1, which modulates the signal into a radio signal according to, for example, a multivalued QAM, and the radio signal is transmitted by the transmitter TX1. On the reception side, the signal MCH1 is received and amplified by the receiver RX1 and is demodulated by the demodulator DEM1 into a digital signal. The digital signal is passed through the switch RSW1 to the interface RBBIF1 and is once stored in an elastic buffer (not shown) in the interface RBBIF1 according to a clock signal CLK' that has been received and regenerated. The same handling is applied to the main signal MCH2. In this way, phase synchronization in the radio transmission section is achieved to realize instantaneous switching to and from the spare channel.
FIG. 15 explains channel switching control according to the prior art.
If the demodulator DEM1 of a reception terminal station detects a radio fault in the current channel MCH1, the detected fault is informed to the switch controller RBSC, which starts processes from step S11. Step S11 checks the spare system PCH (channel, demodulator DEMP, etc.) to see if there is a fault. If there is no fault, step S12 checks the spare system to see if it is used by another current system. If it is not used, step S13 uses the downward channel to send a switching instruction to the switch TSW1 of a transmission terminal station. Step S14 waits for a switching acknowledgment for the switch TSW1 from the transmission terminal station.
Upon receiving the switching instruction, the transmission terminal station switches the switch TSW1 to the spare system in step S1, and step S2 returns a switching acknowledgment related to the switch TSW1 to the reception terminal station.
The switch controller RBSC of the reception terminal station confirms the acknowledgment in step S14 and sends a switching instruction to the switch RSW1 in step S15. Step S16 waits for a switching acknowledgment from the switch RSW1, and upon receiving it, enters a channel switching completed state.
If step S11 finds a fault in the spare system, or if step S12 finds that the spare system is used by another current system, or if step S1 fails to operate the switch TSW1 and step S15 confirms no acknowledgment, or if step S15 fails to operate the switch RSW1 and step S16 confirms no acknowledgment, there will be a channel switching impossible state.
In this way, the system having the N+1 redundant structure must make, if the reception terminal station detects a fault in a current channel, the switch controller RBSC transmit a channel switching instruction to the transmission terminal station only after checking to see, at least, if a spare channel has no trouble in receiving signals and if the spare channel is available.
In this regard, the switch controller RBSC of the reception terminal station may receive the reception fault information directly from the demodulator DEMP, but however, is unable to directly receive the spare channel availability information because it is originally in the switch controller TBSC of the transmission terminal station.
Although there is a way to receive channel control information from the switch controller TBSC, it involves complicated communication control, and there is no guarantee that the channel control information in the transmission terminal station is equal to an actual channel switching state.
Accordingly, the prior art checks to see if the spare channel is available according to the method disclosed below.
FIG. 16 partly shows the structure of the radio transmission system of the prior art and, more precisely, the details of the structure of a part shown in FIG. 14 for determining whether or not a spare channel is available.
The switches TSWP and RSWP of FIG. 14 are required when using the spare channel for an occasional purpose and are not directly related to the following explanation, and therefore, they are omitted.
In the figure, TPNG is a transmitting terminal pattern generator of the transmission terminal station, RPNG is a receiving terminal pattern generator of the reception terminal station, and CMP is a comparator.
The pattern generator TPNG generates dummy random data PN in synchronization with the frame synchronization signal FP of its own station according to the clock signal CLK. The data PN is used to stop a signal level deviating to one side. When a switching signal TSC1 from the switch controller TBSC is 0 to indicate no switching, the switch TSW1 is in a connection state shown in the figure, the main signal MCH1 input to the switch TSW1 is connected to the modulator MOD1, and the data PN from the lower part of the figure (a through line) is connected to the modulator MODP. These conditions are the same for the switch TSW2 which receives the main signal MCH2.
Accordingly, if no channel switching is carried out, the main signals MCH1 and MCH2 are connected to the current channels, respectively, and the dummy random data PN generated by the signal generator TPNG passes through the line that runs through the switches TSW1 and TSW2 and reaches the modulator MODP of the spare system, which adds a frame synchronization signal SYN to the data PN and transmits the same to the spare channel PCH.
The demodulator DEMP of the reception terminal station synchronizes a received frame, regenerates a frame synchronization signal FP' and a clock signal CLK' and, accordingly, reproduces the dummy random data PN. On the other hand, the signal generator RPNG of the reception terminal station independently generates dummy random data PN', which is identical to the received data PN, in synchronization with the frame synchronization signal FP' and according to the clock signal CLK'. The comparator CMP compares the received data PN with the generated data PN' of its own station. If the transmission terminal station carries out no channel switching, PN=PN', and in this case, the comparator CMP provides the switch controller RBSC with a decision signal PDALM=0 to indicate that the spare channel is unused. If the spare channel has been switched to any one of the current channels MCH1 and MCH2, the data PN in the channel MCH1 or MCH2 is not equal to PN', and therefore, the comparator CMP provides the switch controller RBSC with a decision signal PDALM=1 to indicate that the spare channel is in use.
The dummy random signal generators contained in the pattern generator PG and pattern detector PD are generally expensive and must be devices that do not quickly determine disagreement (PN being not equal to PN') on a minor bit error found in the received PN, for the safe management of the system. These factors further complicate the structure of the pattern detector PD and increase the cost of the apparatuses.
A wired SDH system transmits K1 and K2 bytes between transmission and reception terminal apparatuses, to carry out APS (Automatic Protection Switch) control on a current channel that has a fault. In this case, it is also necessary for a reception terminal station to grasp whether or not a spare channel is available in advance. Namely, the present invention is applicable not only to the radio SDH systems but also to wired SDH systems.
In the wired SDH system, a required switching speed is about 50 msec, and therefore, the APS control is not applicable as it is to the radio SDH systems because it is unable to follow the fading speed of the radio SDH systems.
In view of the prior art mentioned above, an object of the present invention is to provide an SDH multiplex transmission system employing a simple structure to allow a reception terminal station correctly determine whether or not a spare channel is used, as well as transmission apparatuses for the system.